This invention relates to a process for the separation of ethane and heavier hydrocarbons from liquefied natural gas, hereinafter referred to as LNG, to provide a volatile methane-rich residue gas stream and a less volatile natural gas liquids (NGL) stream.
As an alternative to transportation in pipelines, natural gas at remote locations is sometimes liquefied and transported in special LNG tankers to appropriate LNG handling and storage terminals. The LNG can then be revaporized and used as a gaseous fuel in the same fashion as natural gas. Although LNG usually has a major proportion of methane, i.e. methane comprises at least 50 mole percent of the LNG, it also contains relatively lesser amounts of heavier hydrocarbons such as ethane, propane, butanes and the like, as well as nitrogen. It is often necessary to separate some or all of the heavier hydrocarbons from the methane in the LNG so that the gaseous fuel resulting from vaporizing the LNG conforms to pipeline specifications for heating value. In addition, it is also often desirable to separate the heavier hydrocarbons from the methane because these hydrocarbons have a higher value as liquid products (for use as petrochemical feedstocks, as an example) than their value as fuel.
Although there are many processes which may be used to separate ethane and heavier hydrocarbons from LNG, these processes often must compromise between high recovery and process simplicity (and hence low capital investment). In U.S. Pat. No. 2,952,984 Marshall describes an LNG process capable of very high ethane recovery via the use of a refluxed distillation column. Markbreiter describes in U.S. Pat. No. 3,837,172 a simpler process using a non-refluxed fractionation column, limited to lower ethane recoveries.
The present invention is generally concerned with the recovery of ethylene, ethane, propylene, propane and heavier hydrocarbons from such LNG streams. It uses a novel process arrangement to allow high ethane recovery while keeping the processing equipment simple and the capital investment low.